1. Field of the Invention
This invention relates to light modules assembled from light emitting diodes (“LEDs”) and to light stings assembled by connecting such light modules together.
2. Description of Related Art
LEDs are increasingly used as light sources in various applications. Some of the features that make LEDs attractive include: low power consumption, long lifespan, low heat generation, small size and weight, robustness, fast switching time and availability in a variety of colors. In addition, in recent years, the cost of making LEDs has significantly decreased, making their use more economical, even in cost-sensitive applications.
One application where LEDs have become particularly popular in recent times has been household and decorative light strings. Such strings are typically formed from between fifty and one hundred LEDs connected together in series. The low power consumption and low heat generation of LEDs make them particularly suitable for such applications, where the cost of power and fire hazard make other types of light such as incandescent lights less attractive.
Nevertheless, there are problems with such strings. Despite their robustness, LEDs do sometimes fail. In the event of failure, the whole light string will go dark, which result is both unattractive and challenging to troubleshoot. The user must then locate the one LED out of fifty or one hundred that is faulty.
In a competing product, strings of incandescent lamps, a popular solution to this problem has been to connect all of the lamps together in parallel to form a string When a lamp in such a parallel circuit fails, the rest of the lamps continue to shine and the defective one is easy to identify and replace.
However, parallel circuitry has not been embraced with LED strings. In contrast to incandescent bulbs that can be built with a filament resistance suitable for parallel connection to a source of alternating current, LEDs are confined by their semiconductor properties to having a forward voltage drop typically in the neighborhood of 1.1 to 3.0 volts. As a result, unless voltage-reduction circuitry is in a light string, a large number of LEDs must be connected together in series to produce a total voltage drop equal to the voltage at which the alternating current is supplied, being 110 VAC in North America. For this reason, complete series-strings of LEDs are sometimes connected together in parallel, but the LEDs themselves are connected together in series to form the string. Thus, there is redundancy between the strings and a user can quickly tell if a string is not working, but it is still a challenge to find the LED within a string that is responsible for a malfunction.
A second reason that parallel circuitry is not seen in LED strings is that there exists a widely held view in the electronic design community that it is bad practice to connect diodes together in parallel with the same polarity. This view is based on the concern that parallel diodes are not well-suited for carrying more current than a single diode can carry on its own, because unless all parallel diodes have identical forward voltage drops, the one with the lowest forward voltage drop will carry the most current, which will cause its temperature to increase, which will cause its forward voltage drop to decrease further, which will cause it to carry even more current until it perhaps fails. If the failing diode fails open, the other parallel diodes will then be forced to carry more current, until they possibly fail one by one. It is important to note nevertheless, that this view seems to have arisen in the context of power circuits that are tasked with delivering high currents through expensive power diodes. In contrast, LEDs typically have significantly less steep current versus voltage curves than other diodes and, consequently, it is less likely that connecting non-identical LEDs in parallel will give rise to significant current differentials and over-heating in one of the LEDs. Furthermore, for typical lower current applications in which LEDs are used, LEDs may be cheap enough to significantly over-specify their rated forward current.
Accordingly, what is needed is a way to provide redundancy in an LED light string, such that when an LED fails, the rest of the light string will still function and the failed LED may be identified without undue difficulty.